Two common approaches currently exist for producing white lighting from LEDs-phosphor-conversion LEDs (pc-LEDs) and discrete color mixing LEDs. In pc-LEDs, a portion of the light emitted from a blue LED chip is down-converted by a phosphor and the longer wavelengths add to the blue to produce white light. In current schemes, blue light (˜460 nm) from an InGaN/GaN-based LED is typically used in combination with a single green-yellow phosphor to produce white light. Although additional phosphors can be added to broaden the emitted spectrum and improve the Color Rendering Index (CRI) and reach warmer color temperatures, it is at a cost to device efficacy. While pc-LEDs currently represent the most popular approach for high intensity white LEDs, they suffer from certain disadvantages, most notably Stokes losses, which are an unavoidable consequence of the energy down-conversion process and result in decreases in LED efficiency. While improvements in phosphor efficiencies continue to be made, significant Stokes losses are inevitable.
Thus, discrete color-mixing LEDs, which incorporate direct-emitting RGB or RYGB elements to create white light, are likely to ultimately offer the best route for highest efficacy white LEDs. However, while high efficiency LEDs exist in the blue wavelengths, realization of high efficiency discrete color mixed RYGB LEDs is seriously hindered by a lack of efficient and temperature stable LEDs in the yellow-red wavelengths (and to a lesser extent in the green). This long wavelength gap is a consequence of the difficulties in forming high quality InGaN films with the higher indium concentrations needed to achieve the longer wavelengths (lower bandgaps) due to lattice mismatch strain effects with GaN. Red LEDs based on the AlInGaP materials system, while efficient at longer red wavelengths, are much less efficient at the shorter red (615 nm) and amber (580-590 nm) wavelengths needed for high efficiency and high CRI color-mixed white LEDs due to limitations in that material system. See M. R. Krames et al., J. Display Technology 3, 160 (2007); and J. M. Phillips et al., Laser & Photonics Reviews 1, 307 (2007). The limitations of poor carrier confinement and carrier losses to indirect parts to the AlInGaP bandstructure are fundamental, and most likely impossible to overcome. Moreover, these limitations of red and yellow AlInGaP LEDs also lead to poor temperature stability and additional efficiency losses and color shifts at operating temperatures typical of high brightness LEDs (e.g. 125° C.).
While currently competitive, pc-LEDs are unable to meet the targets of 81% EQE for green, red, and amber LEDs, which will likely require more efficient direct-emitting LEDs. Current direct-emitting red and amber LEDs based on the AlInGaP materials system suffer from significant efficiency losses and color shifts at the higher operating temperatures typical of high brightness LEDs. Therefore, a need remains for efficient red (610-620 nm) or amber (580-595 nm) LEDs which allow for optimization of spectral efficiency with high color quality over a range of Correlated Color Temperature (CCT) and which also exhibit color and efficiency stability with respect to operating temperature. The present invention is directed to high efficiency amber LEDs which exhibit color and efficiency stability with respect to operating temperature. Amber wavelengths can also provide a path forward for III-nitride based red (615 nm) LEDs based on this invention. This invention therefore enables high efficiency and temperature stable direct emitting LEDs in the amber and red wavelengths necessary for creating high efficiency white light emitters based on color-mixed LEDs.